COS.COS.ForThy

Explanation of sensitivity and recoverability

Physical Factors

The community would be highly intolerant of substratum loss as it is dominated by infaunal and epifaunal species. Removal of the substratum would remove these species. Intolerance has been assessed to be high. Recoverability may be only moderate (see additional information below).

Burrowing species are likely to be able to burrow through the extra layer of smothering sediment and resume their usual infaunal positions, although this would involve an energetic cost. Epifaunal foraminifera may not be able to burrow to the surface and at least a proportion of the population may be lost. However, little information on foraminiferans biology was found, and so it the absence of information an intolerance of high has been recorded, albeit with very low confidence.
The biotope is likely to be more intolerant of smothering by viscous or impenetrable materials e.g. smothering by sediment of a coarser texture may affect burrowing and feeding. Loss of the characterizing species of foraminifera would mean that the biotope is no longer COS.ForThy and so intolerance is high. Recoverability may only be moderate.

Many species in the biotope are either infaunal or deposit feeders that would probably be able to tolerate an increase in suspended sediment at the level of the benchmark. However, there may be additional cleaning costs for Thyasira sp., but this will not affect survival of animals. Some species may benefit from increased food supply if suspended sediment has a high organic content. The intolerance of the biotope is therefore reported to be low. Recovery is likely to be very high as affected animals clean away sediment particles.

A decrease in sedimentation is not likely to affect deposit feeders or foraminifera. Food availability for Thyasira sp. may decline, but this will not affect survival of animals. The intolerance of the biotope has been reported to be low. Recoverability is likely to be moderate.

The community occurs in fine soft mud that only develops in areas of weak tidal streams. Following an increase in water flow rate the surface sediments and epifaunal foraminifera are likely to be winnowed away. The lower substratum inhabited by mature specimens of Thyasira sp., infaunal foraminifera, polychaetes and Amphiura chiajei is likely to remain unchanged. Therefore, since the majority of characterizing species are likely to persist, intolerance has been assessed to be intermediate. On return to normal water flow rates, recoverability is likely to be high.

The biotope occurs in weak tidal streams. A decrease in water flow rate may reduce the supply of particles to suspension feeders in the biotope. However, effects are only expected to be sub-lethal therefore intolerance has been reported to be low. Normal feeding and tube building will resume on return to normal conditions. Recoverability is likely to be very high.

The distribution of fossilised foraminifera is used to track changes in bottom water temperatures, as each species occurs in a particular temperature range (Archer & Martin, 2001). This suggests that they are intolerant of temperature changes.

In the northern North Sea, the COS.ForThy biotope containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria are present in ‘constant Boreal water’ where the bottom temperature among foraminifera communities has been noted to have a low range of variation (Stephen, 1923), with temperatures oscillating between 6 and 8 °C (McIntyre, 1961). Further south, a greater range of bottom temperatures occurs and the biotope is not present. But this could also be due to an increase in sediment particle size with decreasing depth in this area. However, the Atlantic community which contains Thyasira flexuosa and Crithionina granum lives in ‘varying Boreal water’, with temperatures varying between 7 and 13 °C (McIntyre, 1961).

The above evidence suggests that the community is highly dependent on a relatively constant temperature and that different species of Thyasira sp. and foraminifera thrive in different temperature ranges. Intolerance has been assessed to be high. Recoverability is likely to be extremely slow as characterizing species of foraminifera would be lost over a large area (see additional information below).

The distribution of fossilised foraminifera is used to track changes in bottom water temperatures, as each species occurs in a particular temperature range (Archer & Martin, 2001). This suggests that they are intolerant of temperature changes.

In the northern North Sea, the COS.ForThy biotope containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria is present in ‘constant Boreal water’ where the bottom temperature among foraminifera communities has been noted to have a low temperature range (Stephen, 1923), with temperatures oscillating between 6 and 8 °C (McIntyre, 1961). Further south, a greater range of bottom temperatures occurs and the biotope is not present. But this could also be due to an increase in sediment particle size with decreasing depth in this area. However, the Atlantic community which contains Thyasira flexuosa and Crithionina granum lives in ‘varying Boreal water’, with temperatures varying between 7 and 13 °C (McIntyre, 1961).

This suggests that the community is highly dependent on a relatively constant temperature and that different species of Thyasira sp. and foraminifera thrive in different temperature ranges. Intolerance has been assessed to be high. Recoverability is likely to be extremely slow as characterizing species of foraminifera would be lost over a large area (see additional information below).

An increase in turbidity, reducing light availability, may reduce primary production by phytoplankton in the water column. However, productivity in the COS.ForThy biotope is secondary (detritus) and is not likely to be significantly affected by changes in turbidity and so intolerance is assessed as low. On return to normal turbidity levels recovery will be very high as food availability returns to normal.

A decrease in turbidity, increasing light availability, may increase primary production by phytoplankton in the water column. However, productivity in the COS.ForThy biotope is secondary (detritus) and is not likely to be significantly affected by changes in turbidity and so intolerance is assessed as low. On return to normal turbidity levels recovery will be high as food availability returns to normal.

The effects of wave action are attenuated with depth. At a depth of 100 m or more, where the COS.ForThy biotope is present, the effects of an increase in wave exposure would not be felt (Hiscock, 1983) and would probably not have any effect on the characteristic species.

The depths at which the biotope are found means that the community is rarely, if at all, affected by wave disturbance, therefore a decrease in wave exposure is not likely to affect the species within the biotope.

Some species such as polychaetes may respond to vibrations from predators or excavation, however, it is unlikely that noise at the benchmark level would have a detectable effect on the viability of the community. An assessment of not sensitive has been made.

The biotope occurs in deep water where available light is very low. Most species are likely either to have low visual acuity or no mode for detection of visual presence. Therefore, an assessment of not relevant has been made.

Abrasion is likely to damage or result in death of some individuals of the characteristic species of the biotope. For instance, Thyasira sp. are small bivalves, the shells are thin and fragile and abrasion is likely to cause death. Residing 2 cm below the sediment surface means that they are susceptible to abrasive damage. However, some of the impact of physical disturbance will displace individuals without killing them allowing for recovery. Sparks-McConkey & Watling (2001) found that trawler disturbance resulted in a decline of Thyasira flexuosa in Penobscot Bay, Maine. However, the population recovered after 3.5 months. Brittlestars have fragile arms that are likely to be damaged by abrasion or physical disturbance. Amphiura chiajei burrows in the sediment and extends its arms across the sediment surface to feed. Ramsay et al. (1998) suggested that Amphiura sp. may be less susceptible to beam trawl damage than other species of echinoid or tube dwelling amphipods and polychaetes. Brittlestars can tolerate considerable damage to arms and even the disc without suffering mortality and are capable of disc and arm regeneration.

Whilst a proportion of Thyasira sp. and some other species would probably die and other species important within the biotope may be damaged, many individuals would be displaced or suffer damage that can be repaired. Intolerance has been assessed to be intermediate. Recoverability is probably high (see additional information below).

As long as the characterizing species are displaced onto suitable sediment, bivalves, polychaetes and brittlestars would be able to create new burrows and foraminifera would be able to survive.

However, exposure to predators would be increased for a short time and there would be an energetic cost in creating new burrows. Intolerance has been assessed to be low. Recoverability is likely to be high (see additional information below).

Chemical Factors

No information was found concerning the effects of synthetic chemicals on Thyasira sp. or foraminifera, however, other species within the community may be adversely affected.
Dahllöf et al. (1999) studied the long term effects of tri-n-butyl-tin (TBT) on the function of a marine sediment system. TBT spiked sediment was added to a sediment that already had a TBT background level of approximately 27ng/g (83 pool TBT per g) and contained the following fauna: Amphiura spp. and several species of polychaete. Within two days of treatment with a TBT concentration above 13.7 µmol / m² all species except the polychaetes had crept up to the surface and after six weeks these fauna had started to decay. Thus contamination from TBT is likely to result in the death of some intolerant species such as brittle stars. Amphiura chiajei is also known to bioaccumulate PCBs, although direct effects of synthetic chemicals on this species are unknown (Gunnardsson & Skold, 1999). However, Walsh et al., (1986) observed inhibition of arm regeneration in another brittle star, Ophioderma brevispina, following exposure to TBT at levels between 10 ng/l and 100 ng/l.

As no information was found concerning the effects of synthetic chemicals on foraminifera and Thyasira sp., there was insufficient information available to assess intolerance.

No information on the effects of heavy metals on Thyasira sp. or foraminifera were found. There is evidence that polychaetes may be able to adapt to high heavy metal concentrations. For instance, Bryan & Gibbs (1983) presented evidence that Nephtys hombergii from Restronguet Creek possessed increased tolerance to copper contamination. Specimens from the Tamar Estuary had a 96 h LC50 of 250 µg/l, whilst those from Restronguet Creek had a 96 h LC50 of 700 µg/l (35 psu; 13°C). Bryan & Gibbs (1983) suggested that since the area had been heavily contaminated with metals for >200 years, there had been adequate time for metal-resistant populations to develop especially for relatively mobile species. In the short term, however, acute exposure of heavy metals may be deleterious to populations not previously exposed.

Adult echinoderms, such as Ophiothrix fragilis are known to be efficient concentrators of heavy metals including those that are biologically active and toxic (Hutchins et al., 1996). However, there is no information available regarding the effects of this bioaccumulation.

As no information was found concerning the effects of heavy metals on foraminifera and Thyasira sp., there was insufficient information available to assess intolerance.

No information on the effects of hydrocarbons on Thyasira sp. or foraminifera were found. A decrease in brittlestar burrowing activity was recorded at 4,800 and 1,200 ppm total hydrocarbons in sediment (Newton & McKenzie, 1998). However, Newton & McKenzie (1998) suggested that these were a poor predictor of chronic response. Kelly & McKenzie (1995) detected chronic sub-lethal effects around the Beryl oil platform in the North Sea where the hydrocarbon content of the sediment was very low (<3 ppm total hydrocarbons in sediment), and Amphiura chiajei was excluded from areas nearer the platform with higher sediment hydrocarbon content (> 10 ppm). However, the authors did suggest that deleterious effects may also be related to the non-hydrocarbon element of the cuttings such as metals, physical disturbance or organic enrichment.
Amphiura chiajei is also host to symbiotic sub-cuticular bacteria (Kelly & McKenzie, 1995). After exposure to hydrocarbons, loadings of such bacteria were reduced indicating a possible sub-lethal stress to the host (Newton & McKenzie, 1995).

As no information was found concerning the effects of hydrocarbons on foraminifera and Thyasira sp., there was insufficient information available to assess sensitivity.

No information concerning effects of radionuclides on characteristic species within the biotope was found, although adult echinoderms, such as Ophiothrix fragilis are known to be efficient concentrators of radionuclides (Hutchins et al., 1996).
As no information was found concerning the effects of radionuclides on foraminifera and Thyasira sp., there was insufficient information available to assess sensitivity.

Organic enrichment from pulp mills is believed to have been the cause of the death of two Thyasira sp. populations in west Scotland sea lochs (Kunitzer et al., 1992). However, other toxins discharged by the pulp mills could also have been responsible. In addition, similar species such as Thyasira flexuosa may be considerably more tolerant of nutrient enrichment, for example densities of up to 4000 per square metre have been recorded in areas or organic enrichment.

Nilsson (1999) reported a positive response by Amphiura chiajei to increased organic enrichment (27 and 55 g C m², applied four times over eight weeks) demonstrable by an increase in arm tip regeneration rate. In the Skagerrat in the North Sea, Josefson (1990) reported a massive increase in abundance and biomass of Amphiura species between 1972 and 1988 attributable to organic enrichment. Thus increased nutrient availability promoting phytoplankton productivity and an increase in the organic matter reaching the sea bed is likely to be beneficial to Amphiura chiajei.
An increase in nutrients in subtidal habitats of this depth will not cause the biotope to become overgrown with ephemeral algae so the smothering effects often associated with eutrophication will not occur. No information regarding the effects of nutrient enrichment were found. But other characterizing species could potentially benefit from nutrient enrichment. Therefore, on balance not sensitive has been recorded, albeit with very low confidence.

COS.ForThy is a circalittoral biotope that has not been recorded from locations with brackish waters and so is probably highly intolerant of a decrease in salinity.
In the northern North Sea, the COS.ForThy biotope containing Thyasira equalis, Saccammina sp., Psammosphaera sp. and Astrohiza arenaria, is present where salinities remain fairly constant, between 35.20 and 35.26 ppm (McIntyre, 1961).
However, the Atlantic community which contains Thyasira flexuosa and Crithionina granum (foraminifera) occurs in waters where salinity varies between 33.86 and 34.33 (McIntyre, 1961). This suggests that the community is highly dependent on a relatively constant salinity. Mobile species would be able to avoid the change in salinity by moving away, but localised densities would decline.
Amphiura chiajei taken from an area of 24 psu had an LD50 of >21 days for a 70% dilution (17 psu) and an LD50 of 8.5 days for a 50% dilution (12 psu). In comparison, specimens taken from an area with salinity 28.9 psu, had an LD50 of > 12.5 days for a 70% dilution (20 psu) and an LD50 of 6 days for a 50% dilution (14 psu). As Amphiura chiajei is mobile and burrows it may be able to avoid changes in salinity outside its tolerable range.
intolerance has been assessed to be high. Recoverability may be moderate (see additional information below).

Dando & Spiro (1993) found that numbers of Thyasira equalis and Thyasira sarsi decreased rapidly following the deoxygenation of bottom water in the deep basin of Gullmar fjord in 1979-80 (from ~550/m² to~0). However, the abundance of the species increased to approximately 200/m² by 1987.

Polychaetes are burrowing predators in marine sediments that have to survive periods of severe hypoxia and sulphide exposure, while at the same time maintaining agility in order to feed on other invertebrates. Fallesen & Jørgensen (1991) recorded Nephtys hombergii in localities in Århus Bay, Denmark, where oxygen concentrations were permanently or regularly low, but in the late summer of 1982 a severe oxygen deficiency killed populations of Nephtys hombergii and Nephtys ciliata in the lower part of the bay. Such evidence suggests that Nephtys hombergii to be tolerant of short episodes of oxygen deficiency and at the benchmark duration of one week Nephtys hombergii is unlikely to be adversely affected by hypoxic conditions and would revive on return to oxygenated sediment.

Mass mortality in Amphiura filiformis from the south-east Kattegat has been observed during severe hypoxic events (<0.7 mg/l O2), while the abundance of Amphiura chiajei remained unchanged at the same site and time (Rosenberg & Loo, 1988).
In laboratory conditions, Nilsson (1999) maintained specimens of Amphiura chiajei in hypoxic conditions (1.8-2.2 mg O2/l) for eight weeks and recorded no deaths or witnessed specimens escaping to the surface.
This evidence suggests the intolerance of Amphiura chiajei to the benchmark level of 2 mg/l for one week to be low.

Loss of Thyasira sp. would result in loss of the biotope, therefore intolerance has been assessed to be high. Recoverability is likely to be moderate (see additional information below).

Biological Factors

Evidence of the effect of pathogens has only been found for Thyasira gouldi of late. Viral infection of the mutualist bacterium living on the gills of Thyasira gouldi has been suggested as the reason for a major decline in the Loch Etive population. It is likely that similar species of Thyasira sp. would react in the same way. However, it is unlikely that the biotope would be lost entirely. An intolerance of low is suggested but with very low confidence. Recoverability is probably high.

It is extremely unlikely that any of the species indicative of sensitivity would be targeted for extraction. However, benthic trawls or dredging for other species may damage or destroy the shells of Thyasira sp. Other species such as brittlestars may constitute a component of demersal fishing trawl by-catch. Whilst some individuals may die, many more may suffer physical injury.

Thyasira sp. are small bivalves, the shells are thin and fragile and a passing dredge, for example, may cause death. However, some individuals will simply be displaced without killing them. Sparks-McConkey & Watling (2001) found that trawling resulted in the decline of Thyasira flexuosa in Penobscot Bay, Maine. However, the population had recovered within 3.5 months.

Brittlestars have fragile arms that are likely to be damaged by abrasion or physical disturbance. Amphiura chiajei burrows in the sediment and extends its arms across the sediment surface to feed. However, Ramsay et al. (1998) suggested that Amphiura sp. may be less susceptible to beam trawl damage than other species of echinoid or tube dwelling amphipods and polychaetes. Brittlestars can tolerate considerable damage to arms and even the disc without suffering mortality and are capable of disc and arm regeneration.

Whilst a proportion of Thyasira sp. and some other species would probably die and other species important within the biotope may be damaged, many individuals would be displaced or suffer damage that can be repaired. Intolerance has been assessed to be intermediate. Recoverability is probably high (see additional information below).

Additional information

RecoverabilityLittle is known about the mode of reproduction, growth rate and recoverability of foraminifera. In the absence of such information, assessment of recovery potential has to be precautionary and may be more than five years. All other characteristic species within the biotope are fecund and species such as polychaetes and brittlestars are likely to recover fairly quickly.

However, the larval development of Thyasira equalis is lecithotrophic and the pelagic stage is very short or quite suppressed. This agrees with the reproduction of other Thyasira sp., and in some cases (Thyasira gouldi) no pelagic stage occurs at all (Thorson, 1946). This means that larval dispersal is limited. If mortality of Thyasira sp. occurs, there would have to be nearby populations for recovery to occur. Where some individuals survive, due to the fact that larvae spend little or no time in the water column, post-settlement survival may be higher, and the population may be able to recover. It is also possible that adults could be brought into the area by bedload transport, enabling colonization for example:

after a decline in the abundance of Thyasira flexuosa in Penobscot Bay, Maine, after trawler disturbance, populations were reported to recover within 3.5 months (Sparks-McConkey & Watling, 2001);

although deoxygenation of bottom waters between 1979 and 1980, resulted in the depletion of Thyasira equalis and Thyasira sarsi from 550/m² to almost zero, by 1987 200/m² were present (Dando & Spiro, 1993).

Overall, and particularly bearing in mind the lack of information on foraminiferans, recovery of the biotope following catastrophic loss may be only moderate or possibly low.